1. Field of the Invention
This invention relates generally to a method for selectively opening shut-off valves for multiple tanks in a gas feeding system to reduce valve wear and, more particularly, to a method for selectively opening shut-off valves for multiple tanks in a fuel cell system, where the method includes opening the shut-off valve associated with one tank until a pressure equalization occurs in a hydrogen supply line between the opened valve and a pressure drop device, and then opening the other shut-off valves for the other tanks thereafter to reduce valve wear.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
For some vehicle fuel cell designs, hydrogen is stored in compressed gas tanks under high pressure on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the compressed tank can be upwards of 700 bar. The compressed tank typically includes an inner plastic liner that provides a gas tight seal for the hydrogen, and an outer carbon fiber composite layer that provides the structural integrity of the tank. Because hydrogen is a very light and diffusive gas, the inner liner must be carefully engineered in order to act as a permeation barrier. The hydrogen is removed from the tank through a pipe. At least one pressure regulator is provided that reduces the pressure of the hydrogen within the tank to a pressure suitable for the fuel cell system.
Typically, the fuel cell systems discussed above include several compressed hydrogen tanks that are coupled to a common hydrogen supply line to the stack, where each tank includes a primary tank shut-off valve. A second shut-off valve or other flow control device is provided downstream of the tank shut-off valves, where the device can be a secondary shut-off valve, a flow restrictor, a pressure regulator, etc. The tank shut-off valves are designed to have a maximum number of cycles before the wear and tear of the valve components reduces the valves effectiveness, including sealing integrity against leaks. Typically, this maximum number of cycles is dependent upon the pressure difference between the inlet side of the valve and the outlet side of the valve when it is opened and closed. Particularly, if the pressure on the downstream side of the tank shut-off valve is much lower than the pressure in the tank during an opening cycle, the valve will wear out more quickly. It is desirable to limit the number of cycles of the tank shut-off valve at these pressure differences to increase the life of the valve.